1. Field of the Invention
The present invention relates to a method and to a system for accessing of transmission channels of a communication system with at least a central base station, with at least a duplex transmission channel and with a plurality of subscriber stations independent of each other, where each subscriber station can have access to the duplex transmission channel according to a multiple access method.
2. Brief Description of the Background of the Invention Including Prior Art
A system for transmission of messages via radio communication can be recited as an example for such a communication system. A radio transmission system is known from German Patent Application Laid Open DE-OS No. 2,537,683 having stationary radio stations and mobile subscriber stations. There, to each stationary radio station (central base station) is assigned a set of duplex transmission channels. One of the channels, the control channel, is employed for the transmission of control information for a system of operations and the other channels (the voice channels) are employed for voice communication. In the following, the transmission direction from the subscriber station to the central base station is called forward direction and the reverse transmission direction is called backward direction. The principles of operation of various channel access methods are taught in German Patent Application Laid Open DE-OS No. 2,537,683.
An identification of a subscriber station can be for example, a calling number which is in correspondence to a telephone number. For example, the local central office can assign a number to the subscriber. Then a city code can be assigned to the local station. Further, an area code can be provided for the area in which the local station is disposed. In addition, the area can be located in a certain country, and a country code can be provided for this purpose. In addition, the identification assigned can comprise a certain signal identifying the type of service to which this station and network belongs. It can be recognized that there are a variety of options in characterizing stations and subscribers.
An access cell information is shorter than a complete identification and comprises a signal which has less than 80 percent of the number of character units of a comparable identification signal set forth above for the type of service common in that area. Preferably, an access signal does not employ more than 50 percent of the characterizing units required for a full identification of a subscriber station. It is particularly advantageous when the access signal does not cover more than 30 percent of the transmission characterizing units required for a full identification information of that station. For example, an access signal can comprise from about 2 to 10 and preferably from about 3 to 5 character units employed for this type of information transmission. A character unit can be for example on octal number, a hexadecimal number, a character of an international telegraph alphabet, an ASCII character, a byte, a memory word or the like.
At the start of each transmission from a mobile subscriber station to a conventional central base station, initially as identification of the mobile subscriber station is transmitted in the forward direction via the control channel (duplex transmission channel). If two or more subscriber stations emit their identification at the same time onto the control channel, then none of these identifications can be recognized in the central base station except for the special case, where one of the identifications is received with a substantially larger power than from all the others.
Also, with an increasing number of subscriber stations, the probability increase that more than one of the subscriber stations intend to access the control channel at the same time. Since the control channel is to be used by all subscriber stations jointly, the probability for access attempts to the control channel at the same time and therewith the probability of destruction of the signals are particularly large.
A further increase in the probability of destruction results from the repetition of attempts of accessing based on identifications which are not recognized. With each destruction of an access, the transmission channel is not useable for at least the duration of an identification, that means, it is blocked. The portion of time the control channel is blocked due to access destructions increases additionally with the length of the identification, that is, with the number of the subscriber stations to be admitted to the complete communications system.
In case of short and frequent transmissions the capacity of the duplex transmission channel can only be utilized in small part.
Joel, Jr. teaches in U.S. Pat. No. 3,663,762, a mobile communication system which incorporates an electronic processor. The system comprises a plurality of base stations each located in individual cell areas. Predetermined cell areas are periodically interrogated to detect movement of located mobile stations into new areas.
Wells et al. in U.S. Pat. No. 3,898,390 teach a mobile radio communication system and a method for increasing the effective use of communication channels in a small zone communication system. Calls between two mobile units may be routed through a central control terminal and base stations. The mobile units may all search for and lock onto an appropriate control signal channel. A call is initiated over a base station control signal channel and the control and signalling link connecting the base station to the control terminal. An available voice channel assigned to the base station through which the cell is established is then assigned to the cell. In the event that the cell was originated by a mobile unit and a cell back is requested, the central processing unit may store the cell until circuits are clear and then notify the mobile unit of the availability of circuits at a later time (column 7, lines 59 to 63).
U.S. Pat. No. 3,590,166 and U.S. Pat. No. 3,458,664 teach conventional systems which exemplify many of the structural items employed according to the present invention. Reference is made to these patents as illustrating the various elements useful in the context of the present invention.